Interior Minister Matteo Salvini said there were around 30 dead and many injured in a serious condition

Italy was in shock Tuesday after dozens of people were killed when a motorway bridge collapsed in heavy rain in the Italian city of Genoa in what the government called an "immense tragedy".

The collapse, which saw a vast stretch of the A10 freeway tumble onto railway lines in the northern port city, came as the bridge was undergoing maintenance work and as the Liguria region, where Genoa is situated, experienced torrential rainfall.

"Unfortunately there are around 30 dead and many injured in a serious condition," Interior Minister Matteo Salvini told reporters.

Prime Minister Giuseppe Conte later said the official count stood at 25 dead but that the toll could rise as rescuers searched through the tangled wreckage.

Italian President Sergio Mattarella said a "catastrophe" had hit Genoa and the whole of Italy, as attention turned to what might have caused the collapse and who might be ultimately responsible.

"Italians have the right to modern and efficient infrastructure that accompanies them safely through their everyday lives," Mattarella said in a statement.

Conte added: "It's clear that we need to immediately plan special measures. We need to launch a monitoring plan for all infrastructure and we must not allow another tragedy like this to happen again."

Rescuers scouring through the wreckage, strewn among shrubland and train tracks, said there were "dozens" of victims, as rescue helicopters winched survivors on stretchers from the ruined bridge.

Cars and trucks were tangled in the rubble and nearby buildings damaged by vast chunks of concrete, according to an AFP photographer at the scene.

"We're not giving up hope, we've already saved a dozen people from under the rubble," a fire official, Emanuele Giffi, told AFP.

"We're going to work round the clock until the last victim is secured."

As cars and trucks tumbled off the bridge, Afifi Idriss, 39, a Morrocan lorry driver, just managed to come to a halt in time.

"I saw the green lorry in front of me stop and then reverse so I stopped too, locked the truck and ran," he told AFP.

The green lorry was still on the bridge late evening, stopped just short of the now yawning gap.

The incident -- the deadliest of its kind in Europe since 2001 -- is the latest in a string of bridge collapses in Italy, a country prone to damage from seismic activity but where infrastructure generally is showing the effects of a faltering economy.

- 'Unacceptable' way to die -

Aerial footage showed more than 200 metres (650 feet) of the viaduct, known locally as the Morandi bridge, completely destroyed.

"I'm following with the utmost apprehension what is happening in Genoa and what looks like it could be an immense tragedy," Transport and Infrastructure Minister Danilo Toninelli said on Twitter.AFP / ANDREA LEONI A fire service spokesperson told AFP that the bridge had mostly fallen on rail tracks several dozen metres below and that cars and trucks had fallen with the rubble

Salvini, who is also leader of the nationalist League party in the coalition government, vowed to hold those responsible for the disaster accountable.

"I have gone over this bridge hundreds of times, and I commit to digging and finding out who is responsible for an unacceptable tragedy, because it's not possible that in 2018 you can work and die in these conditions," he said.

The cause of the disaster was not immediately clear, although weather services in the Liguria region had issued a storm warning Tuesday morning.

The national motorways body said on its website that "maintenance works were being carried out on the base of the viaduct", adding that a crane had been moved on site to assist the work.